What was found
DURING CLIMB, A STRONG OIL SMELL WAS PRESENT THROUGH OUT THE AIRCRAFT. FLIGHT CREW FOLLOWED SMOKE REMOVAL PROCEDURES, BUT ODOR COULD NOT BE ISOLATED. FLIGHT RETURNED TO DEPARTURE AIRPORT LANDING WITHOUT FURTHER INCIDENT. GROUND PERSONNEL REMOVED AND REPLACED ALL THREE WATER SEPARATOR BAGS AND DETERMINED THAT APU WAS SOURCE OF ODOR. APU PLACED ON MEL.